This invention relates generally to current source circuitry and more particularly to current source circuitry having relatively high output impedances.
As is known in the art, current sources have a wide range of applications in linear integrated circuits. One such current source, a so-called "Wilson current source", is described in an article entitled "A Monolithic Junction FET n-p-n Operational Amplifier" by George A. Wilson in IEEE Journal of Solid-State Circuits, December 1968. Such current source improves on a conventional current source (which has a transistor with a diode coupled between its base and emitter to provide a current flow in the collector of the transistor substantially equal to a reference current fed to the junction of the diode and the base of such transistor) by adding a second transistor having its base coupled to the collector of the first transistor and its emitter connected to the junction of the diode and the base of the first transistor. With such arrangement, the current in the collector of the second transistor is substantially equal to a reference current passing to the junction of the collector of the first transistor and the base of the second transistor.
While this so-called "Wilson current source" is useful in a wide variety of applications, in some applications it is desirable that the current source have a relatively high output impedance, as where such current source is to be used with other transistors to provide current mirrors which "track" or "mirror" the current produced by the current source. The desirability of increasing the output impedance of the current source is to reduce the variations produced by the current source with variations in supply voltage.